Fair Value (Fair Value On Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	$ 74.9	$ 71.6	$ 67.5	$ 62.6
Transfers into Level 3	6.9	3.3	10.6	16.8
Transfers out of Level 3	(9.3)	(2.7)	(15.8)	(4.7)
Included in earnings	0.1	0.1	(0.4)	(0.1)
Included in other comprehensive income	(2.1)	0.8	(0.9)	2.5
Purchases	7.3	1.4	19.1	5.1
Sales	(13.2)	(2.8)	(15.5)	(10.5)
Ending balance	103.3	71.7	103.3	71.7
Chaucer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	38.7		38.7
Chaucer [Member] | Corporate [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	0.1		0.1
Chaucer [Member] | Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	5.6		5.6
Chaucer [Member] | Asset-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	8.8		8.8
Chaucer [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	14.5		14.5
Chaucer [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Chaucer acquisition	24.2		24.2
Corporate [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	36.8	31.2	28.2	28.9
Transfers into Level 3	6.9	3.3	10.6	9.9
Transfers out of Level 3	(8.8)	(2.7)	(15.3)	(2.7)
Included in earnings	0.1		0.1	0.1
Included in other comprehensive income	(0.3)	0.3		1.3
Purchases		0.4	11.8	0.7
Sales	(12.3)	(0.3)	(13.0)	(6.0)
Ending balance	22.5	32.2	22.5	32.2
Residential Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	0.5	1.1	0.8
Transfers out of Level 3	(0.5)		(0.5)
Purchases				1.4
Sales	(0.1)	(0.1)	(0.4)	(0.4)
Ending balance	5.5	1.0	5.5	1.0
Commercial Mortgage-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	5.1	5.9	5.5	6.2
Purchases	7.3		7.3
Sales	(0.2)	(0.2)	(0.6)	(0.5)
Ending balance	12.2	5.7	12.2	5.7
Municipal [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	16.1	14.9	16.6	15.5
Included in other comprehensive income		0.1	0.1	0.3
Sales	(0.3)	(0.4)	(0.9)	(1.2)
Ending balance	15.8	14.6	15.8	14.6
Asset-Backed [Member] | Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	13.5	15.7	13.5	9.2
Transfers into Level 3				6.9
Transfers out of Level 3				(2.0)
Included in earnings		0.1		0.1
Included in other comprehensive income	(0.2)	0.4	0.1	0.6
Purchases				2.0
Sales	(0.3)	(1.8)	(0.6)	(2.4)
Ending balance	21.8	14.4	21.8	14.4
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	72.0	68.8	64.6	59.8
Transfers into Level 3	6.9	3.3	10.6	16.8
Transfers out of Level 3	(9.3)	(2.7)	(15.8)	(4.7)
Included in earnings	0.1	0.1	0.1	0.2
Included in other comprehensive income	(0.5)	0.8	0.2	2.2
Purchases	7.3	0.4	19.1	4.1
Sales	(13.2)	(2.8)	(15.5)	(10.5)
Ending balance	77.8	67.9	77.8	67.9
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance	2.9	2.8	2.9	2.8
Transfers out of Level 3
Included in earnings			(0.5)	(0.3)
Included in other comprehensive income	(1.6)		(1.1)	0.3
Purchases		1.0		1.0
Ending balance	$ 25.5	$ 3.8	$ 25.5	$ 3.8